linux-kernel - Re: [PATCH] KVM: move struct kvm_vcpu * array to the bottom of struct kvm

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Date:   Mon, 8 Nov 2021 15:35:11 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Juergen Gross <jgross@...e.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: move struct kvm_vcpu * array to the bottom of
 struct kvm

On Sat, Nov 06, 2021, Nicholas Piggin wrote:
> Excerpts from Sean Christopherson's message of November 6, 2021 12:49 am:
> >> It would next be possible to now make this a dynamically sized array,
> >> and make the KVM_MAX_VCPUS more dynamic
> > 
> > Marc has a mostly-baked series to use an xarray[1][2] that AFAICT would be well
> > received.  That has my vote, assuming it can get into 5.16.  Marc or Juergen,
> > are either of you actively working on that?
> > 
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/vcpu-xarray
> > [2] https://lkml.kernel.org/r/871r65wwk7.wl-maz@kernel.org
> 
> Seems like a good idea if it can allow vcpu structs to be allocated to 
> preferred nodes.
> 
> >> however x86 kvm_svm uses its own scheme rather than kvm_arch for some reason.
> > 
> > What's the problem in kvm_svm?
> 
> It embeds a struct kvm so it couldn't be variable sized.

Oooh, when you said "dynamically sized" I thought you meant

	struct kvm_vcpu *vcpus;

...

	kvm->vcpus = kcalloc(...);

Anyways, SVM and VMX are quite different despited both being x86, to keep them
separated without requiring an extra allocation, kvm_svm and kvm_vmx embed kvm.
Ditto for vcpu_vmx and vmx_svm.  Obviously not a hard requirement, but there also
hasn't been a reason not to do embed kvm/kvm_vcpu.